Is Your Solar System Actually Underperforming? A Homeowner's Guide to Checking, Using One Simple Number

October 9, 2026

Solar panels are quiet, they have no moving parts, and they sit somewhere you never look. That combination makes them brilliantly low-maintenance - and it also means a system can lose 15% of its output for two years running and nobody notices.

The good news is that checking takes about ten minutes, once a year, and needs exactly one number. No tools, no roof ladder, no engineer.

The one number: kWh per kWp

Every solar system has a nameplate size in kWp - kilowatt-peak. It is on your paperwork, and it is usually the panel count multiplied by the panel wattage. Twelve 430-watt panels is 5.16 kWp.

Over a year, your system produces some number of kWh - kilowatt-hours. Your inverter app knows this, and so does your generation meter.

Divide one by the other and you get your specific yield:

Annual kWh ÷ system kWp = kWh/kWp per year

This is the number professionals use, because it strips out system size entirely. A 4 kWp system and a 10 kWp system on the same street should land on roughly the same kWh/kWp. It lets you compare your roof against a benchmark rather than against your neighbour's bigger array.

What "normal" looks like

Specific yield depends overwhelmingly on where you live, then on orientation, tilt and shading. Broad ranges for a reasonably well-sited system:

Location	Typical annual kWh/kWp
Northern Germany	850 - 1,000
Southern Germany (e.g. Munich area)	950 - 1,100^[2]
Scotland and northern England	750 - 900
Southern England	850 - 1,000
Northern Italy	1,100 - 1,300
Central and southern Italy	1,300 - 1,600

Across Germany as a whole, 900 to 1,100 kWh/kWp is the normal band.^[3] PVGIS - the free calculator from the European Commission's Joint Research Centre - returns values from roughly 800 to 1,800 kWh/kWp across Europe depending on location.^[4]

An east-west split roof will sit maybe 10 to 15% below these figures, and that is completely normal rather than a fault. A partly shaded roof will sit lower again.

A worked example. A 6 kWp system near Stuttgart produced 5,400 kWh last year. That is 5,400 ÷ 6 = 900 kWh/kWp. For southern Germany that is at the low end of normal - probably fine if the roof faces east-west or has some shading, but worth a closer look if it is a clear south-facing roof.

Where to find your numbers

Annual kWh. Your inverter's app or web portal is the easiest source - SolarEdge, Fronius, SMA Sunny Portal, Enphase Enlighten, Huawei FusionSolar and the rest all show yearly totals. Failing that, read your generation meter on the same date each year and subtract.

System kWp. On your installation certificate, your grid registration (Marktstammdatenregister in Germany, MCS certificate in the UK, GSE paperwork in Italy), or calculated from panel count × panel watts ÷ 1,000.

If you have per-panel monitoring - Enphase microinverters and SolarEdge optimisers both give this - you get a bonus. Panels in the same string should track each other closely through the day. One panel consistently below its neighbours is the clearest fault signal you will ever get^[5], and it shows up on a phone screen.

If your number looks low, work through this in order

Start with the free and obvious, not the alarming.

1. Compare against the right benchmark. Was last year unusually dull? Most inverter portals let you compare year-on-year. A 5% swing between years from weather alone is entirely ordinary.

2. Account for degradation. Panels lose output slowly and predictably - most warranties allow for a small annual decline, typically well under 1% a year after the first year. A ten-year-old system producing around 95% of its original output is behaving exactly as designed. This is not a fault.

3. Look for shade that has grown. This is the single most common real cause. Trees grow. Neighbours build extensions and dormers. A satellite dish or new flue gets fitted. Shading that did not exist when the system was designed can quietly cost you a lot, and the fix is sometimes as cheap as a conversation with a tree surgeon. Go and look at the roof at 10am and again at 4pm, in winter when the sun is low.

4. Check whether the panels are actually dirty. In most of Germany and the UK, rain does the cleaning and the effect of soiling is small. In drier parts of Italy, or anywhere near a busy road, farmland, or under trees that drop sap and bird traffic, it is more significant. A visibly grimy array is worth cleaning - by someone with the right equipment, not you with a hose from a ladder.

5. Check the inverter for errors. Inverters log faults and often display them without anyone ever reading them. A string that has dropped offline can halve output while everything looks superficially fine. Check the app for error codes and, if you have string-level data, whether both strings are producing.

6. Check the clipping. If your inverter is smaller than your array - a common and often sensible design choice - output flatlines at the inverter's limit on the brightest days. That flat top on a sunny-day graph is normal and usually costs very little annually.

7. Then call someone. If the number is more than about 15% below benchmark and steps 1 to 6 have not explained it, that is a real service call. Check your warranty first: panel performance warranties typically run 25 years, inverters more like 5 to 12, and the installer's workmanship warranty is usually separate again.

The habit worth building

Two entries in a note on your phone, once a year, on the same date:

Meter reading or annual kWh total
The kWh/kWp figure you calculated

After three years you have your own baseline, which is far more useful than any national average. A gradual drift tells you about degradation or growing shade. A sudden drop tells you something broke, and roughly when.

Ten minutes a year, and you will know about a fault in months rather than in years.

Not installed yet? Get the benchmark first

If you are still at the deciding stage, the useful move is to know what your roof should produce before anyone quotes you. That way an over-optimistic estimate stands out immediately - and you will have the number to check against once the panels are up.

Our free Solar Roof Planner gives you exactly that. Enter your address, trace your roof on a satellite image, and it estimates panel count, system size, yearly output, savings and payback for your actual roof, orientation and location. Free, no sign-up, about two minutes.

Keep the estimate. In a year's time it becomes your yardstick.